Systems and Methods for Improving the Efficiency of Combined Cascade and Multicomponent Refrigeration Systems

ABSTRACT

Systems and methods for improving the efficiency of combined cascade and multicomponent refrigeration systems by utilizing one or more ejectors to reduce and/or eliminate compression stages. The systems and methods change the temperature profile, which reduces the energy consumption of both the mixed refrigeration system and the pre-cooling system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No. 62/872,317, filed Jul. 10, 2019, which is incorporated herein by reference. This application, U.S. Provisional Application No. 62/872,318, and U.S. Provisional Application No. 62/885,958, which are incorporated herein by reference, are commonly assigned to Bechtel Oil, Gas and Chemicals, Inc.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to systems and methods for improving the efficiency of combined cascade and multicomponent refrigeration systems. More particularly, the systems and methods improve the efficiency of combined cascade and multicomponent refrigeration systems by utilizing one or more ejectors to reduce and/or eliminate compression stages.

BACKGROUND

The natural gas liquefaction process takes natural gas, primarily comprised of methane at high pressure and passes it through consecutive refrigeration cycles. These refrigeration cycles can be single or multi-component. The present disclosure relates to multi-component refrigeration cycles. Two examples of such processes are presented in FIGS. 1 and 2.

In FIG. 1, a schematic diagram illustrates a conventional propane, pre-cooling system with mixed refrigerant liquefaction (hereinafter collectively the “C3MR system”). Feed Gas 102 enters the system and is mixed with boil-off gas from line 104 that is recompressed. The feed gas 102 is chilled by means of three consecutive heat exchangers (106, 108, 110). The heat exchangers chill the feed gas to a temperature of approximately 23° F. in line 111. The chilled feed gas in line 111 is then distributed through two zones 112, 114 of a spiral-wound design heat exchanger, reaching a temperature of—about 262° F. in line 116.

The propane pre-cooling system is comprised of a three-stage compressor 118, three flash drums 120, 122, 124 and a chiller 126 to reject heat. Vapor propane is introduced into the first compression stage of compressor 118 and is compressed through three successive stages before being transferred through line 128 to the chiller 126. The chiller 126, typically an air cooler or cooling water exchanger, chills the compressed propane to a temperature of about 100° F. The outlet pressure in line 130 is equivalent to the pressure at which the refrigerant is liquid, which for propane is approximately 190 psia. The liquid propane in line 130 is flashed through an expansion valve 132 to a pressure of approximately 80 psia and a temperature of about 41° F. It is then introduced into the first heat exchanger 106 to pre-chill the feed gas 102. The outlet of the first heat exchanger 106 consists of a two-phase mixture of liquid and vapor propane. The mixture is flashed in the high stage of flash drum 124. The vapor is recompressed in the compressor 118. The liquid is flashed to a lower pressure through a second expansion valve 134 to a temperature of approximately 25° F. and 61 psia and is introduced into the second heat exchanger 108. The outlet of the first heat exchanger 108 consists of a two-phase mixture of liquid and vapor propane. The mixture is flashed in the middle stage flash drum 122. The vapor is recompressed in the compressor 118. The liquid is flashed to a lower pressure through a third expansion valve 136 to a temperature of approximately −35° F. and 18 psia and is introduced into the third heat exchanger 110. The propane is completely vaporized in the third exchanger 110 and is distributed into a compressor suction drum 120.

The mixed refrigerant system consists of a two-stage compressor 138, a refrigerant chiller 140 for heat rejection, a flash drum 142 for separating the mixed refrigerant into liquid and vapor phases and the spiral wound heat exchanger with two zones, 112 and 114. Vapor mixed refrigerant is introduced to the two-stage compressor 138. The mixed refrigerant composition is variable and is designed to closely match the cooling curve of the feed gas 102. In this example, the mixed refrigerant composition is described in Table 1 below.

TABLE 1 Component Mole Fraction H₂O 0.000 CO₂ 0.000 Nitrogen 0.100 Methane 0.400 Ethane 0.350 Propane 0.150 i-Butane 0.000 n-Butane 0.000 i-Pentane 0.000 n-Pentane 0.000 n-Hexane 0.000 Ethylene 0.000 Propene 0.000

The mixed refrigerant is compressed to a pressure of approximately 601 psia and is then chilled to a temperature of 118° F. by the mixed refrigerant chiller 140. The mixed refrigerant chiller 140 can typically be an air cooler or cooling water exchanger. The chilled mixed refrigerant in line 144 is introduced into the second and third heat exchangers 108, 110 and is subsequently chilled to about −30° F. At this state, the refrigerant consists of a liquid and vapor mixture in line 146. In this example, the vapor molar fraction is approximately 43%. The refrigerant is separated in flash drum 142 into liquid and vapor refrigerants and is then inserted into the top and bottom sections of the spiral wound heat exchanger. The refrigerant is then collected at the bottom of the spiral wound exchanger 112 and sent back to the suction of the two-stage compressor 138.

Liquefied gas in line 116 is flashed to atmospheric pressure via an expansion valve 148 as well as via the line losses in the transfer pipe and stored in a cryogenic liquefied natural gas (LNG) tank. Because the liquefied gas is subcooled, no vapor is generated. Boil-off gas from the LNG tank is recompressed to pipeline pressure via a boil-off gas compressor 150 and chiller 152. The chiller is typically an air cooler or cooling water exchanger.

In FIG. 2, a schematic diagram illustrates a conventional multi-component, integrated, propane pre-cooling system with single mixed refrigerant liquefaction (hereinafter collectively the “IPSMR system”). Feed Gas 202 enters the system and is mixed with boil-off gas from line 204 that is recompressed. The feed gas 204 is chilled by means of three consecutive heat exchangers (206, 208, 210). The heat exchangers chill the feed gas to a temperature of approximately −35° F. The chilled feed gas is then distributed through a brazed aluminum heat exchanger 212, reaching a temperature of about −265° F. at the outlet in line 216.

The propane pre-cooling system is comprised of a three-stage compressor 218, three flash drums 220, 222, 224 and a chiller 226 to reject heat. Propane in the vapor phase is introduced into the first compression stage of compressor 218 and is compressed through three successive stages before being transferred through line 228 to the chiller 226. The chiller 226, typically an air cooler or cooling water exchanger, chills the compressed propane to a temperature of about 100° F. The outlet pressure in line 230 is equivalent to the pressure at which the refrigerant is liquid, which for propane is approximately 190 psia. The liquid propane in line 230 is flashed through an expansion valve 232 to a pressure of approximately 80 psia and a temperature of about 41° F. It is then introduced into the first heat exchanger 206 to pre-cool the feed gas 202. The outlet of the first heat exchanger 206 consists of a two-phase mixture of liquid and vapor propane. The mixture is flashed in the high stage flash drum 224. The vapor is recompressed in the compressor 218. The liquid is flashed to a lower pressure through a second expansion valve 234 to a temperature of approximately 25° F. and 61 psia and is introduced into the second heat exchanger 208. The outlet of the first heat exchanger 208 consists of a two-phase mixture of liquid and vapor propane. The mixture is flashed in the middle stage flash drum 222. The vapor is recompressed in the compressor 218. The liquid is flashed to a lower pressure through a third expansion valve 236 to a temperature of approximately −35° F. and 18 psia and is introduced into the third heat exchanger 210. The propane is completely vaporized in the third exchanger 210 and is distributed into a compressor suction drum 220.

The mixed refrigeration system consists of a brazed aluminum heat exchanger 212, three flash drums 242, 244, 246, a two-stage mixed refrigerant compressor 238, a mixed refrigerant chiller 240, and a mixed refrigerant pump 248. Vapor mixed refrigerant 250 is introduced into the mixed refrigerant compressor 238. The mixed refrigerant composition is variable and is designed to closely match the cooling curve of the feed gas 202. In this example, the mixed refrigerant composition is described in Table 2 below.

TABLE 2 Component Mole Fraction H₂O 0.0000 CO₂ 0.0000 Nitrogen 0.1115 Methane 0.2903 Ethane 0.4008 Propane 0.0000 i-Butane 0.0000 n-Butane 0.1520 i-Pentane 0.0453 n-Pentane 0.0000 n-Hexane 0.0000 Ethylene 0.0000 Propene 0.0000

The mixed refrigerant is compressed to a pressure of approximately 718 psia and is then chilled via the mixed refrigerant chiller 240 to a temperature of about 95° F. At this state, the mixed refrigerant is approximately 70% vapor in line 252. The vapor and liquid mixed refrigerant is then transferred to a flash drum 246. The vapor and liquid are transferred through various sections of the brazed aluminum heat exchanger 212 and flashed through three separate let-down valves 254, 256, 258. The refrigerant is partially condensed in the brazed aluminum heat exchanger 212 and is then returned to flash drum 242. Liquid from the flash drum 242 is transferred to the middle stage flash drum 244 via a pump 248. The middle stage flash drum 244 operates at a pressure of approximately 196 psia and a temperature of about 95° F. Liquid from the flash drum 244 is transferred to the brazed aluminum heat exchanger 212 and recycled back to flash drum 242. Vapor from the middle stage flash drum 244 is compressed in the mixed refrigerant compressor 238.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to the accompanying drawings, in which like elements are referenced with like reference numbers, in which:

FIG. 1 is a schematic diagram illustrating a conventional C3MR system.

FIG. 2 is a schematic diagram illustrating a conventional IPSMR system.

FIG. 3 is a schematic diagram illustrating one embodiment of the present disclosure retrofitted in a pre-existing liquefied natural gas process.

FIG. 4 is a schematic diagram illustrating one embodiment of the present disclosure applied to a C3MR liquefied natural gas process.

FIG. 5 is a schematic diagram illustrating another embodiment of the present disclosure applied to an IPSMR liquefied natural gas process.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

The subject matter of the present disclosure is described with specificity, however, the description itself is not intended to limit the scope of the disclosure. The subject matter thus, might also be embodied in other ways, to include different structures, steps and/or combinations similar to and/or fewer than those described herein, in conjunction with other present or future technologies. Although the term “step” may be used herein to describe different elements of methods employed, the term should not be interpreted as implying any particular order among or between various steps herein disclosed unless otherwise expressly limited by the description to a particular order. Other features and advantages of the disclosed embodiments will be or will become apparent to one of ordinary skill in the art upon examination of the following figures and detailed description. It is intended that all such additional features and advantages be included within the scope of the disclosed embodiments. Further, the illustrated figures and dimensions described herein are only exemplary and are not intended to assert or imply any limitation with regard to the environment, architecture, design, or process in which different embodiments may be implemented. The pressures and temperatures described herein thus, illustrate exemplary advantages and/or parameters of the various embodiments.

In one embodiment, the present disclosure includes a system for chilling a feed gas, which comprises: i) a first heat exchanger enclosing a first portion of a feed gas line and a portion of a first chilled refrigerant line; ii) a first flash drum in fluid communication with the first chilled refrigerant line for receiving a two-phase refrigerant from the first heat exchanger, the first flash drum having a first vapor outlet line and a first liquid outlet line; iii) a second heat exchanger enclosing a second portion of the feed gas line and a portion of a second chilled refrigerant line; iv) a second flash drum in fluid communication with the second chilled refrigerant line for receiving a two-phase refrigerant from the second heat exchanger, the second flash drum having a second vapor outlet line and a second liquid outlet line; v) a third heat exchanger enclosing a third portion of the feed gas line and a portion of a third child refrigerant line; vi) a drum in fluid communication with the third chilled refrigerant line for receiving a vaporized refrigerant from the third heat exchanger, the drum having a drum vapor outlet line; vii) an ejector in fluid communication with the drum vapor outlet line, the first chilled refrigerant line, and a compressed refrigerant line; and vii) a compressor in fluid communication with the first vapor outlet line, the second vapor outlet line, and the compressed refrigerant line connected to a chiller for chilling a compressed refrigerant in the compressed refrigerant line

In another embodiment, the present disclosure includes a method for chilling a feed gas, which comprises: i) introducing a feed gas stream through a first heat exchanger, a second heat exchanger and a third heat exchanger; ii) chilling the feed gas stream in the first heat exchanger by circulating a first chilled refrigerant stream adjacent the feed gas stream in the first heat exchanger using a compressor and a chiller to convert a first vapor refrigerant stream from a first flash drum into a liquid refrigerant stream and an ejector to convert the liquid refrigerant stream into the first chilled refrigerant stream; iii) chilling the feed gas stream in the second heat exchanger by circulating a second chilled refrigerant stream adjacent the feed gas stream in the second heat exchanger using a first liquid refrigerant stream from the first flash drum; iv) chilling the feed gas stream in the third heat exchanger by circulating a third chilled refrigerant stream adjacent the feed gas stream in the third heat exchanger using a second liquid refrigerant stream from the second flash drum; v) transferring a vapor refrigerant stream from the third heat exchanger to a drum; and vi) returning at least a portion of the vapor refrigerant stream in the drum to the ejector for lowering the temperature of the first chilled refrigerant stream.

Referring now to FIG. 3, a schematic diagram illustrates one embodiment of the present disclosure retrofitted in a pre-existing liquefied natural gas process. A vaporized refrigerant from the lowest stage drum 120 is taken through line 302 to an ejector 304 that is preferably a liquid motive ejector. The motive for the ejector 304 is supplied via line 130 and is passed at saturated liquid conditions through a high-efficiency pump 306. The propane chilling compressor 118 can comprise three stages or can employ two stages of compression and instead redirect the total flow of vaporized refrigerant from the lowest stage drum 120 through line 302. This facilitates a significant decrease in mass flow to the compressor 118, as depicted in Table 3 below, based on a HYSYS™ simulation. As a result, capacity in the propane chilling system is increased, facilitating the change in temperature profile. The adjustment of the temperature profile differs by implementation, but generally is a reduction of about 2° F. to about 4° F. in the feed gas stream 102 and about 5° F. to about 100° F. in the supplemental refrigeration system 312. The supplemental refrigeration system 312 produces one of a chilled feed gas stream and a liquified feed gas stream in line 116. The supplemental refrigeration system 312 may be a mixed refrigeration system that includes a mixed refrigerant.

TABLE 3 Prior Art FIG. 4 Refrigeration Equipment Mass Flow Mass Flow Loop Tag % Difference % Difference Mixed K-6000.I Base −11.66% Refrigerant 138 K-6000.II Base −11.66% Propane 118 K-3001.I Base — K-3001.II Base −52.48% K-3001.III Base −10.86%

Due to the fact that the chilled feed gas stream or the liquified feed gas stream in line 116 is subcooled, the boil-off gas recompression system can be eliminated in favor of another liquid motive ejector 310 that is controlled by means of the letdown valve 148. Pressure in the form of vapor suction through line 308 to the ejector 310 is monitored to ensure that the LNG tank does not reach vacuum pressure. A small temperature increase of approximately 3-5° F. is noted from HYSYS simulation models, but due to the significant subcooling of the chilled feed gas stream or the liquified feed gas stream at the letdown valve 148, no vapor generation occurs.

Referring now to FIG. 4, a schematic diagram illustrates one embodiment of the present disclosure applied to a C3MR liquefied natural gas process. In this embodiment, the lowest compression stage from the drum 120 to the compressor 118 is eliminated. The entire vaporized refrigerant from drum 120 is thus, diverted through line 302 to the liquid motive ejector 304. The resultant effect is a reduction in the temperature in line 111 from about 23° F. to about 19° F. The temperature of the mixed refrigerant in line 146 is also reduced from about −30° F. to about −34° F. As a result, the vapor fraction in flash drum 142 is adjusted from about 43% to about 41%. The inter-stage flashes conditions in the supplemental refrigeration system improve from about −162° F. in the conventional C3MR liquefied natural gas process to about −190° F. The temperature remains the same. Table 4 (below) illustrates the impact of this embodiment applied to a natural gas liquefaction process for two cases, modeled using HYSYS™. One case maintains the natural gas feed rate to the natural gas liquefaction terminal. The second case increases the feed rate to maintain the compressor 118 at a capacity like a conventional C3MR liquefied natural gas process. An observed feed rate increase of nearly 17% is depicted when the terminal is revamped with the present embodiment. Additionally, a brake power reduction of nearly 22% is observed.

TABLE 4 FIG. 4 (w/increased Prior Art FIG. 4 feed rate and revamp) Brake Power hp % Difference Base −21.96% −11.98% Feed Rate MMtpa % Difference Base 0.00% 16.57% Feed Temperature ° F. Value 60.00 60.00 60.00 Product Rate MMBtu/hr % Difference Base 1.82% 18.82% MMtpa % Difference Base 0.33% 18.42% Thermal Efficiency % % 92.75 94.34 94.52 UA Btu/hr-° F. % Difference 0.00% 48.13% 564.08%

Referring now to FIG. 5, a schematic diagram illustrates another embodiment of the present disclosure applied to an IPSMR liquefied natural gas process. In this embodiment, the lowest compression stage from the drum 220 to the compressor 218 is eliminated. The entire vaporized refrigerant from drum 220 is thus, diverted through line 302 to the liquid motive ejector 304. Because of the relatively small size of the propane chilling system in this embodiment, compared to the embodiment in FIG. 4, the increased capacity in the propane chilling system is used to cool the mixed refrigerant in line 501. The mixed refrigerant passed through heat exchangers 206, 208 and 210 is chilled to a temperature of about −13° F. in line 502. The mixed refrigerant is then re-introduced into the brazed aluminum heat exchanger 212. Table 5 (below) illustrates the impact of this embodiment applied to a natural gas liquefaction process for two cases, modeled using HYSYS™. One case only modifies the propane chilling system. The second case modifies both the propane chilling system and the mixed refrigeration system depicted in FIG. 5. Additionally, a brake power reduction of nearly 12% is observed.

TABLE 5 FIG. 5 (w/MR Prior Art FIG. 5 integration) Brake Power hp % Difference Base −1.52% −11.54% Feed Rate MMtpa % Difference Base 0.00% 0.00% Feed Temperature ° F. Value 95 95 95 Product Rate MMBtu/hr % Difference Base 0.11% 0.81% MMtpa % Difference Base 0.11% 0.81% Thermal Efficiency % % 93.35 93.45 94.10 UA Btu/hr-° F. % Difference 0.00% 1.68% 221.98%

The systems and methods disclosed herein thus, improve the efficiency of combined cascade and multicomponent refrigeration systems by utilizing one or more ejectors to reduce and/or eliminate conventional compression stages. The systems and methods change the temperature profile, which reduces the energy consumption of both the mixed refrigeration system and the pre-cooling system.

While the present disclosure has been described in connection with presently preferred embodiments, it will be understood by those skilled in the art that it is not intended to limit the disclosure to those embodiments. For example, the present disclosure may be implemented in the mixed refrigeration systems described herein and other multi-stage refrigeration processes for chilling a feed gas, such as other cascade refrigeration cycles and mixed refrigerant cycles, to achieve similar results. Although propane is used as an exemplary refrigerant for the pre-cooling system, it is not intended to preclude other refrigerants from being used instead of propane. It is therefore, contemplated that various alternative embodiments and modifications may be made to the disclosed embodiments without departing from the spirit and scope of the disclosure defined by the appended claims and equivalents thereof. 

1. A system for chilling a feed gas, which comprises: a first heat exchanger enclosing a first portion of a feed gas line and a portion of a first chilled refrigerant line; a first flash drum in fluid communication with the first chilled refrigerant line for receiving a two-phase refrigerant from the first heat exchanger, the first flash drum having a first vapor outlet line and a first liquid outlet line; a second heat exchanger enclosing a second portion of the feed gas line and a portion of a second chilled refrigerant line; a second flash drum in fluid communication with the second chilled refrigerant line for receiving a two-phase refrigerant from the second heat exchanger, the second flash drum having a second vapor outlet line and a second liquid outlet line; a third heat exchanger enclosing a third portion of the feed gas line and a portion of a third child refrigerant line; a drum in fluid communication with the third chilled refrigerant line for receiving a vaporized refrigerant from the third heat exchanger, the drum having a drum vapor outlet line; an ejector in fluid communication with the drum vapor outlet line, the first chilled refrigerant line, and a compressed refrigerant line; and a compressor in fluid communication with the first vapor outlet line, the second vapor outlet line, and the compressed refrigerant line connected to a chiller for chilling a compressed refrigerant in the compressed refrigerant line.
 2. The system of claim 1, further comprising a first expansion valve positioned between the first heat exchanger and the ejector for producing a chilled refrigerant in the first chilled refrigerant line.
 3. The system of claim 2, further comprising a second expansion valve positioned between the second heat exchanger and the first flash drum for producing a chilled refrigerant in the second chilled refrigerant line.
 4. The system of claim 3, further comprising a third expansion valve positioned between the third heat exchanger and the second flash drum for producing a chilled refrigerant in the third chilled refrigerant line.
 5. The system of claim 1, further comprising a pump positioned between the chiller and the ejector for pumping the chilled compressed refrigerant in the first chilled compressed refrigerant line.
 6. The system of claim 1, further comprising a supplemental refrigeration system in fluid communication with the feed gas line and an outlet line for one of a chilled feed gas and a liquified feed gas.
 7. The system of claim 6, wherein the supplemental refrigeration system includes a mixed refrigerant.
 8. The system of claim 1, wherein the compressor is in fluid communication with the drum vapor outlet line.
 9. The system of claim 6, further comprising: another ejector in fluid communication with the outline line and a boil-off gas line connected to a tank for holding boil-off gas.
 10. The system of claim 9, further comprising a let-down valve positioned in fluid communication with the outlet line for controlling flow of the chilled feed gas or the liquified feed gas in the outlet line to the another ejector.
 11. A method for chilling a feed gas, which comprises: introducing a feed gas stream through a first heat exchanger, a second heat exchanger and a third heat exchanger; chilling the feed gas stream in the first heat exchanger by circulating a first chilled refrigerant stream adjacent the feed gas stream in the first heat exchanger using a compressor and a chiller to convert a first vapor refrigerant stream from a first flash drum into a liquid refrigerant stream and an ejector to convert the liquid refrigerant stream into the first chilled refrigerant stream; chilling the feed gas stream in the second heat exchanger by circulating a second chilled refrigerant stream adjacent the feed gas stream in the second heat exchanger using a first liquid refrigerant stream from the first flash drum; chilling the feed gas stream in the third heat exchanger by circulating a third chilled refrigerant stream adjacent the feed gas stream in the third heat exchanger using a second liquid refrigerant stream from the second flash drum; transferring a vapor refrigerant stream from the third heat exchanger to a drum; and returning at least a portion of the vapor refrigerant stream in the drum to the ejector for lowering the temperature of the first chilled refrigerant stream.
 12. The method of claim 11, further comprising expanding the first chilled refrigerant stream before circulating the first chilled refrigerant stream in the first heat exchanger.
 13. The method of claim 12, further comprising expanding the second chilled refrigerant stream before circulating the second chilled refrigerant stream in the second heat exchanger.
 14. The method of claim 13, further comprising expanding the third chilled liquid refrigerant stream before circulating the third chilled refrigerant stream in the third heat exchanger.
 15. The method of claim 11, further comprising pumping the liquid refrigerant stream from the compressor to the ejector.
 16. The method of claim 11, further comprising chilling the feed gas stream in a supplemental refrigeration system to produce one of a chilled feed gas stream and a liquified feed gas stream.
 17. The method of claim 16, wherein the supplemental refrigeration system includes a mixed refrigerant.
 18. The method of claim 11, further comprising returning at least a portion of the vapor refrigerant stream in the drum to the compressor.
 19. The method of claim 16, further comprising: recompressing a boil-off gas stream from a boil-off gas tank by transferring the one of the chilled feed gas stream and the liquified feed gas stream to another ejector; and directing at least a portion of the boil-off gas stream through the another ejector.
 20. The method of claim 19, further comprising controlling a flow of the chilled feed gas stream or the liquified feed gas stream to the ejector with a let down valve. 